The invention relates to the preparation of alkoxycinnamic esters (alkoxycinnamates).
2-Ethylhexyl 4-methoxycinnamate and 3-methylbutyl 4-methoxycinnamate and isomers thereof are known and efficient light protection agents for the UV-B region and are prepared on an industrial scale.
To prepare these products, processes are sought which are low-cost and can be carried out with large yields.
U.S. Pat. No. 5,527,947 describes a process in which C1-C4-alkoxybenzaldehydes, such as anisaldehyde, and C1-C4-alkyl acetates, such as methyl acetate, are dissolved in an inert hydrocarbon, such as heptane, toluene or petroleum ether. In the presence of a strongly alkaline metal base, such as sodium methoxide, the feed substances react to give a mixture of the corresponding C1-C4-alkyl-C1-C4-alkoxy cinnamates, the alkali metal salts of the corresponding C1-C4-alkoxy cinnamic acid and the C1-C4-alkanols. Following acidification of the reaction mixture with a strong polybasic acid, such as sulfuric acid, the liberated acetic acid is esterified with the C1-C4-alkanoles and distilled off. The mixture of C1-C4-alkyl-C1-C4-alkoxycinnamates and C1-C4-alkoxycinnamic acids which remains is reacted with a C5-C14-alkanol in the presence of a metal salt of a strong polybasic acid, such as sulfuric acid. Following transesterification and esterification, the corresponding C5-C14-alkyl-C1-C4-alkoxycinnamate, e.g. 2-ethylhexyl-4-methoxycinnamate, is obtained with 83 to 87% yield.
We have found a process for the preparation of cinnamic esters of the formula 
in which
R1 is an alkyl group having 1 to 4 carbon atoms and
R2 is 2-ethylhexyl or 3-methylbutyl,
which comprises condensing acetic C5-C8-esters of the formula 
in which R2 has the meaning given above,
with an alkoxybenzaldehyde of the formula 
in which R1 has the meaning given above,
in the presence of an alkali metal alkoxide, and removing alcohol which form during the reaction.
The process according to the invention can be illustrated by the following equation: 
Alkoxybenzaldehydes for the process according to the invention may, for example, be anisaldehyde, 4-ethoxybenzaldehyde, 4-propoxybenzaldehyde, 4-iso-propoxy-benzaldehyde, 4-butoxybenzaldehyde and 4-iso-butoxybenzaldehyde.
The acetic C5-C8-esters for the process according to the invention are 2-ethylhexyl acetate or 3-methylbutyl acetate.
Alkali metal alkoxides for the process according to the invention are preferably the sodium and potassium alkoxides of lower aliphatic alcohols (C1 to about C4), such as methanol, ethanol, propanol, isopropanol, butanol and isobutanol. Particular preference is given to sodium methoxide.
The acetic C5-C8-esters can be used in pure form or in a mixture with 2-ethylhexanol or 3-methylbutanol. In order to increase the economic feasibility of the process, it is advantageous to prepare 2-ethylhexyl acetate or 3-methylbutyl acetate before or during the condensation reaction in situ, and to use this reaction mixture without purification or washing for the condensation reaction.
This may be carried out, for example, by
a) transesterifying 2-ethylhexanol or 3-methylbutanol with an acetic C1-C4-ester, such as methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate or isobutyl acetate, in the presence of an alkali metal alkoxide and/or alkaline earth metal alkoxide to give the feed material 2-ethylhexyl acetate or 3-methylbutyl acetate, or
b) esterifying 2-ethylhexanol or 3-methylbutanol with acetic acid autocatalytically or in the presence of catalytic amounts of a strong acid, such as sulfuric acid or 4-toluenesulfonic acid, and subjecting the mixture to incipient distillation.
The mixture prepared under a) can immediately be taken with a C1-C4-alkoxybenzaldehyde, such as anisaldehyde, to the condensation reaction.
The mixture prepared under b) is prepared for the condensation reaction by adding alkali metal alkoxide.
The alkoxybenzaldehyde is preferably added in the temperature range from xe2x88x9210 to 120xc2x0 C., particularly preferably from 10 to 30xc2x0 C., over the course of from 0.5 to 5 hours, particularly preferably 1 to 2 hours.
After-stirring is carried out for 0.5 to 10 hours, preferably 1 to 3 hours, the reaction temperature being increased to 50 to 150xc2x0 C., preferably 80 to 110xc2x0 C. During the after-stirring, a vacuum of from 800 mbar to 2 mbar, preferably from 80 mbar to 40 mbar, is applied.
The alcohol which forms during the process according to the invention is preferably separated off during distillation in a vacuum.
In the after-stirring period under vacuum, residual alkoxybenzaldehyde is reacted to give the 4-alkoxycinnamic ester, and lower homologous 4-methoxycinnamates are transesterified to give the alkoxycinnamic ester according to the invention, 2-ethylhexyl 4-methoxycinnamate or 3-methylbutyl 4-methoxycinnamate.
The reaction mixture is then preferably admixed with a strong acid, such as sulfuric acid, sulfuric acid/NaHSO4 or 4-toluenesulfonic acid. The mixture is then distilled up to a bottom temperature of 150xc2x0 C., the 4-methoxycinnamic acid (about 20%) which forms as byproduct being esterified to give the alkoxycinnamic ester according to the invention. At the same time, acetic acid which has formed is esterified to give 2-ethylhexyl acetate or 3-methylbutyl acetate. After washing and distillation, 2-ethylhexyl 4-methoxycinnamate or 3-methylbutyl 4-methoxycinnamate is obtained with high yield (90 to 93%) and high purity (96 to 98%) in a simple distillation.
The intermediate fractions 2-ethylhexanol/2-ethylhexyl acetate or 3-methylbutanol/3-methylbutyl acetate which form during the distillation can be co-used again in the next condensation reaction.
The advantages of the process according to the invention are:
Use of low-cost raw materials
Minimizing of waste materials and reuse of secondary streams
Minimizing of organic contaminants in waste water
It is surprising that alkoxycinnamic esters can be prepared by the process according to the invention in high yields and in high purity by simple distillation.